When you wear sari at night cant sleep And when only bra can sleep?

When you wear a "night sari", you can't sleep. When you wear only a "bra", you can sleep.

The pun here is on the words "night sari" and "bra". "Night sari" sounds similar to "nightie" (a nightgown), and "bra" sounds similar to "brassiere" (a type of undergarment). So, the joke is that when you wear a nightie, you can't sleep, and when you wear a brassiere, you can sleep.